This invention relates to measuring systems, and particularly to measuring systems using strain gauges to determine the stress or strain placed upon a member.
Strain gauges are made of materials whose resistances vary in response to deformation of its shape, by tension, compression, or torsion. However, the resistances change only slightly in response to substantial deformation. In fact, the resistance of a material can be expected to change only 0.1% as it is stressed without the gauge material being permanently deformed. Therefore, it has been the practice to connect two pairs of gauges whose resistances change in opposite directions in response to the same strain, into a bridge and to sense the differences in the changes in resistance in response to the strain. However, the simultaneous use of four strain gauges with interdependent resistance relationships results in a complex expensive and often unreliable mechanical arrangement.
Replacing two of the gauges in the bridge with constant resistors relieves the problem of mechanical complexity. However, because the gauges and the resistors respond differently to changes in temperature, the resulting output is highly temperature sensitive. This is particularly so if the 0.1% resistance change throughout the elastic limit of the gauge material must be divided into 1000 units or more. In that case, a temperature induced change of 10.sup.-6 of the total gauge resistance produces a change of one unit. A minor difference in the temperature coefficient of resistance between the resistors and the gauges can effect a subtantial variation in the bridge output. Moreover, this alteration shifts the point at which a zero strain is measured. Effectively, the resistors must exhibit a stringent precision of 1 to 2 parts per million per degree centigrade.